The present invention relates to a piston for an internal combustion engine, and more specifically relates to a piston where deformation of the skirt in a thrust and thrust-opposing direction of the piston is suppressed.
The structure of a piston of a prior art is shown in FIGS. 7 and 8. As shown in FIGS. 7 and 8, a piston 1 is constructed of aluminum alloy for the purpose of weight reduction. Piston 1 includes a crown 2 and a skirt 3. A pair of bosses 4 are formed at an axial mid-portion of skirt 3 and a piston-pin 5 is inserted in bosses 4 and extend over the paired bosses 4. A connecting rod 6 is rotatably coupled with piston-pin 5. Piston-ring grooves 7 are formed in a radially outer portion of crown 2 and the lowermost groove functions as an oil-ring groove 8. A recess means 9 is formed in a upper portion of crown 2 and recess means 9 constitutes one portion of a combustion chamber of the internal combustion engine.
A radially outer portion of crown 2 where piston-ring grooves 7 are formed constitutes a ring-land 10. The diameter of the outer surface of ring-land 10 is formed slightly smaller than that of skirt 3 to prevent stick with a cylinder bore because ring-land 10 is located near to the combustion chamber and becomes hotter than skirt 3 resulting in larger thermal expansion than skirt 3. Since portions 3b of skirt 3 where bosses 4 are located receive a large amount of heat conduction and become at high temperatures, the portions 3b of skirt 3 expand to a greater extent than portions 3a of skirt 3 which are located at positions offset circumferentially by 90 degrees from portions 3b. To prevent stick with the cylinder bore, a distance between the outside surfaces of the opposed portions 3b are constructed smaller than a distance between the outside surfaces of the opposed portions 3a.
When piston 1 having the above-mentioned structure is slidably inserted in the cylinder bore and is rotatably coupled with connecting rod 6, piston 1 pivots around the axis of bosses 4 in a direction perpendicular with the axis of bosses 4 according to the swinging motion of connecting rod 6. Portions 3a of skirt 3 which are on sides perpendicular with the axis of bosses 4 and are opposed to each other mainly contact the cylinder bore and receive thrust and thrust-opposing forces from the cylinder bore whereby the orientation of piston 1 is kept. It is very important to maintain a clearance between the inside surface of the cylinder bore and the portions 3a of skirt 3 which are on thrust and thrust-opposing sides of piston 1, that is, which are on sides adjacent to the A-A axis of piston 1 in FIG. 8. This clearance is very important in maintaining the posture of piston 1 and must be set at an appropriate value. If the clearance is too small, skirt 3 would bind as it expands thermally. If the clearance is too large, slapping sounds would occur during the reciprocating motion of piston 1 and sounds in a car room would be increased.
Further, slits 11 are formed in piston 1 on the thrust and thrust-opposing sides of oil-ring groove 8. Slits 11 extend from the thrust and thrust-opposing portions of piston 1 toward the portions of piston 1 which are circumferentially offset by 90 degrees from the thrust and thrust-opposing portions of piston 1. In such a piston having slits 11, the portions of ring-land 10 which are located above bosses 4 produce a large thermal expansion because there are no slits above bosses 4 and therefore, heat can conduct from crown 2 to bosses 4 and because the amount of heat conduction is large due to the large heat capacity of bosses 4. To suppress the thermal expansion of the thrust and thrust-opposing sides of piston 1, struts 12 which circumferentially extend from the portions offset by 90 degrees from the thrust and thrust-opposing sides including bosses 4 toward the thrust and thrust opposing sides of piston 1 are provided in a radially inner portion of piston 1. Struts 12 are constructed of a metal which has smaller thermal expansion characteristics than an aluminum alloy. In such a manner thermal expansion of piston 1 is suppressed.
However, when piston 1 having slits 11 and struts 12 reciprocates in the cylinder bore and an inertia force acts on piston 1, piston-pin 5 elastically deforms such that both end portions of piston-pin 5 are bent upward with respect to the axial center portion of piston-pin 5 where piston-pin 5 is connected with the connection rod. Due to the deformation of piston-pin 5, the opposed portions of skirt 3 where bosses 4 are located deform radially outside at the lower portions thereof and the diameter of the opposed portion is increased while the diameter of the thrust and thrust-opposing sides of skirt 3 is decreased which will produce slapping sounds. Such a deformation of skirt 3 easily occurs especially in piston 1 having slits 11, because the upper portions of the thrust and thrust-opposing sides of skirt 3 is not restricted due to slits 11 and skirt 3 can easily deform. Repetition of such a deformation produces a permanent deformation of skirt 3 in such an order that a permanent radial deformation of about 50 micron will occur after one hundred hours test. Therefore, suppression of deformation of skirt 3 has been strongly desired.
Japanese Utility Model Publication SHO No. 58-32150 discloses ribs for suppressing deformation of a skirt. The rib disclosed in the publication is a rib bent in the form of an arc along a circumferentially extending inside surface of the skirt and has little rigidity in the thrust and thrust-opposing direction of the piston. Therefore, the rib has little effect in suppressing a deformation of the skirt in the thrust and thrust-opposing direction of the piston. Additionally, since the rib is provided at an axial mid-portion of the skirt, the rib would increase a radially inward deformation of the lower portion of the skirt when the upper portion of the skirt expands radially outward, because ribs would function as a fixed point of the deformation. Therefore, provision of such a rib would increase slapping sounds.